The present invention relates generally to an etching process for etching a semiconductor wafer, and more particularly to a process which co-relates etch depth directly with dopant diffusion depth of a doped single crystal silicon wafer.
In the manufacture of semi-conductor devices, dry etching is an indispensable process to the formation of IC patterns on a semi-conductor wafer. Dry etching is the removal of surface material from a wafer through use or a chemical process. Various types of dry etching are known. One method is a plasma etch process in which a thin film is placed on a semi-conductor wafer. A photoresist is placed on a segment of the semi-conductor wafer to protect the thin layer. The wafer is placed in a reaction chamber between a pair or, parallel plate electrodes. Reactant gasses and high RF power are introduced, generating a plasma. The plasma is applied to the semi-conductor wafer, thereby removing material from the top layer of the semi-conductor wafer not protected by the photoresist. The photoresist is removed, exposing the protected segment.
In order to determine when a semi-conductor device is at an ideal height for fabrication purposes, it is necessary to detect an etch endpoint. In a typical method or detecting the etch endpoint, a luminous intensity or plasma etching is monitored, and the endpoint of the plasma etching is determined from the change in the luminous intensity. If the etching process is continued after the material to be removed has been completely removed from the semi-conductor water, the surface of the wafer will be unnecessarily etched, resulting in a poor quality semi-conductor device. Therefore, it is essential that the endpoint of the etching process be detected with high accuracy.
One conventional method of etch endpoint detection normalizes the luminous intensity over a particular time period. An endpoint is detected when a signal level representing a current value of luminous intensity drops below a preset fraction of the normalized luminous intensity value. Normal values for the present fraction range from 1/10 (10%) to 1/2 (50%). Detection in this matter, therefore, requires a large and abrupt decrease in luminous intensity over successive time periods. Large and abrupt changes in luminous intensity are generally only possible when the thin film and the underlying semi-conductor wafer have significantly crystal structure and/or significantly different chemical compostions. If the crystal structure or chemical composition of the two materials are not significantly different, little or no change in luminous intensity will occur, resulting in an inconsistent etch end point detection.
Another conventional method of etch endpoint detection includes a single crystal semi-conductor wafer. This method is accomplished without the use of a thin film. Etching is performed in a similar manner and in a similar environment as in the thin film method. A photoresist is placed on a segment of the semi-conductor wafer. The semi-conductor wafer is placed in a reaction chamber between a pair of parallel plate electrodes. Reactant gasses and high RF power are introduced, generating a plasma. The plasma is applied to the semi-conductor wafer, thereby removing material from the top layer of the semi-conductor wafer not protected by the photoresist. The photoresist is removed, exposing the protected segment. In this method, however, the luminous intensity of the plasma etching does not significantly change as the etch progresses. Capturing large luminous intensity signal level drops is trade more difficult when a thin film is not used. The change in the material's chemical composition is less distinguishable, making the change in luminous intensity signal levels less distinguishable. The more dependent an etch is on ion bombardment, the less selective it will be between different materials. Thus, detecting an etch endpoint through on a single crystal wafer is more difficult. Low luminous intensity signal levels are inherent to single crystal etches. When accompanied by normal process variations and parameter drift, these low signal levels are virtually indistinguishable from noise. These low signal levels result, thus, in low repeatability. In this process, repeatability refers to the detection consistency of luminous intensity signal levels from wafer to wafer.
In summary, problems exist with both prior methods of dry etching. In the thin film method, the use of a thin film requires a substantial change in crystal structure and/or chemical composition of the underlying semi-conductor wafer. Also, detecting an endpoint based on normalized luminous intensities over time intervals requires abrupt signal level drops over successive time periods. This process produces inconsistent results and can result in etching further into the underlying semi-conductor wafer then desired, thereby reducing overall etch quality. In the single crystal method, low signal levels are inherent to the process, making the luminous intensity signal levels difficult to distinguish from noise. Because a single crystal method does not result in substantial changes of crystal structure or chemical composition, detecting a change in luminous intensity is inconsistent, resulting in low repeatability. The trade-off, therefore, is between signal level and overall etch quality. Higher signal levels can be achieved only at the expense of etch profile.
It is evident that there is a continuing need for an improved etch endpoint detection method for a single crystal silicon in modern IC processes. Conventional endpoint techniques are not adequate for a single crystal silicon wafer which requires both repeatably controlling single crystal silicon etches and maintaining etch quality. In particular, there is a need for high repeatability while maintaining high etch quality.